1. Field of the Invention
The present invention relates to a gas sensor for measuring the concentration of a gas component such as NOx gas or combustible gas (e.g., HC or CO) in exhaust gas emitted from an internal combustion engine of a vehicle, such as an automobile, ship, or airplane, or from an industrial combustion engine, or in combustion gas emitted from, for example, a boiler.
2. Description of the Related Art
Recently, with exhaust gas regulations being tightened, studies have been conducted on engine control and catalyst control realized through direct measurement of the concentration of NOx, HC, or CO gas in exhaust gas emitted from, for example, an engine. An NOx gas concentration sensor for such an application is disclosed in, for example, SAE paper No. 960334, pp. 137-142, 1996. The NOx gas concentration sensor assumes the form of a laminate of solid electrolyte layers, each formed of a zirconia sheet, and includes a first diffusion passage, a first cavity portion, which communicates with the atmosphere under measurement via the first diffusion passage, a second diffusion passage, and a second cavity portion, which communicates with the first cavity portion via the second diffusion passage. The sensor further includes a first oxygen pump cell and an oxygen sensor cell, which is exposed to the interior of the first cavity portion, and a second oxygen pump cell, which is exposed to the interior of the second cavity portion. The oxygen sensor cell is adapted to measure oxygen concentration in the first cavity portion. On the basis of the measured oxygen concentration, the first oxygen pump cell pumps out oxygen from the first cavity portion, thereby diffusing a gas having a controlled oxygen concentration into the second cavity. A predetermined voltage is applied to a pair of electrodes of the second oxygen pump cell, causing NOx to dissociate into ions on one of the paired electrodes that is exposed to the interior of the second cavity portion. The thus-generated oxygen ions pass through the solid electrolyte that constitutes the second oxygen pump cell. As a result, a limiting current flows between the paired electrodes. On the basis of the limiting current, NOx gas concentration is determined. The paired electrodes of the second oxygen pump cell are disposed such that one electrode is exposed to the interior of the second cavity portion, while the other electrode is exposed to the atmosphere.
According to the above-described conventional gas sensor, oxygen concentration is lowered in the first cavity portion, and the concentration of NOx in the gas diffused into the second cavity portion is determined according to a limiting-current process. Since a detection output (current flowing between the paired electrodes of the second oxygen pump cell) with respect to gas to be measured (hereinafter referred to as xe2x80x9cgas under measurementxe2x80x9d) is very small (of the order of several xcexcA), accurate measurement of such a small current is difficult. In order to detect such a small current, a sensor unit must be of high precision and thus becomes expensive. Also, the structure of the gas sensor becomes complex; specifically, the first and second cavity portions, the first and second oxygen pump cells, and the oxygen sensor cell are provided independently of one another.
In view of the foregoing, an object of the present invention is to provide a gas sensor that produces a large gas sensor output even with respect to a low-concentration gas to be detected and that has a simple structure.
Aspects of the present invention are described below. First aspect: a cavity portion, whose oxygen concentration is controlled at a constant level; an active electrode having a relatively high catalytic capability with respect to NOx or combustible gas; an inactive electrode having a relatively low catalytic capability with respect to NOx or combustible gas; and an oxygen concentration cell, which is disposed so as to be exposed to the interior of the cavity portion. Second aspect: the active electrode and the inactive electrode are disposed so as to be exposed to the interior of the same cavity portion. Third aspect: the active electrode contains one or more elements from the platinum group, which includes Pt, Rh, Pd, Ir, and Ru; and the inactive electrode contains one or more elements selected from the transition metals, which include Au, Ni, Co, Cr, Fe, Mn, Cu, Ti, and Zn, so that the catalytic capability with respect to NOx or combustible gas becomes lower than that of the active electrode. Fourth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; and the oxygen-concentration-sensing electrode and the inactive electrode are implemented in the form of a common electrode. Fifth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; the oxygen concentration reference electrode and the inactive electrode are implemented in the form of a common electrode; and the common electrode is disposed outside the cavity portion. Sixth aspect: an oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion in order to detect oxygen concentration in the cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; and the oxygen-concentration-sensing electrode and the active electrode are implemented in the form of a common electrode. Seventh aspect: The cavity portion includes a first cavity portion and a second cavity portion, which communicates with the first cavity portion across a diffusion resistance and to which the oxygen concentration cell is exposed; an oxygen-concentration-sensing electrode, which is exposed to the interior of the first cavity portion in order to detect oxygen concentration in gas that diffuses from the first cavity portion into the second cavity portion; an oxygen concentration reference electrode, which generates an electric potential that serves as a reference for the oxygen-concentration-sensing electrode; an oxygen pump cell, which is exposed to the interior of the first cavity portion and pumps out oxygen from and/or pumps oxygen into the first cavity portion on the basis of the differential in electric potential between the oxygen-concentration-sensing electrode and the oxygen concentration reference electrode; and the active electrode and the inactive electrode are disposed within the second cavity portion. Eighth aspect: NOx or combustible gas concentration is determined by means of the oxygen concentration cell, which is exposed to the interior of the cavity portion whose oxygen concentration is held constant.
Features of preferred embodiments of the present invention will next be described. Preferably, a gas sensor according to the present invention assumes a laminate structure composed of thin sheets of solid electrolyte. An oxygen concentration cell includes an active electrode, an inactive electrode, and an oxygen-ion-conductive solid electrolyte layer on which the active and inactive electrodes are formed. The active and inactive electrodes have a reversible catalytic function (catalytic function related to oxygen dissociation) in relation to at least a dissociation reaction of oxygen molecules for injecting oxygen into the solid electrolyte layer and a recombination reaction of oxygen to cause the solid electrolyte layer to release oxygen. Preferably, in order to hold constant oxygen concentration in a cavity portion, to the interior of which the oxygen concentration cell is exposed, the oxygen pump cell is disposed so as to be exposed to the interior of the cavity portion. By holding the oxygen concentration in the cavity portion constant, the oxygen concentration dependency of an electromotive force (gas sensor output) that is generated by a concentration cell effect in the oxygen concentration cell can be reduced, which oxygen concentration cell is exposed to the interior of the cavity portion. That is, among components of a gas sensor output, an offset corresponding to oxygen concentration (partial pressure of oxygen) in the cavity portion is reduced, thereby reducing variation in gas sensor output caused by temperature variation and abrupt variation in oxygen concentration in the subject gas.
The oxygen pump cell includes an oxygen-ion-conductive solid electrolyte layer and a pair of electrodes formed on the solid electrolyte layer, and is exposed to the interior of the cavity portion to which the oxygen concentration cell is exposed, or to a space that communicates with the cavity portion. The oxygen pump cell is constructed such that upon applying voltage to the paired electrodes, the oxygen pump cell pumps out oxygen from and/or pumps oxygen into the cavity portion or the space communicating with the cavity portion. Preferably, the oxygen pump cell is controlled so as to pump out oxygen according to the difference in electric potential between the oxygen-concentration-sensing electrode, which is exposed to the interior of the cavity portion, and the oxygen concentration reference electrode, which is exposed to an atmosphere of constant oxygen concentration. Specifically, voltage applied to the paired electrodes of the oxygen pump cell is controlled such that the oxygen concentration in the cavity portion attains such a constant, low level as to have substantially no effect on measurement of the concentration of a predetermined gas. In the case of detection of NOx, NO2 may be decomposed in the cavity portion by the oxygen pump cell. Also, a portion of NO may be decomposed in the cavity portion by the oxygen pump cell. The amount of decomposed NO can be compensated by means of, for example, oxygen pump current flowing through the oxygen pump cell.
In order to detect the concentration of NOx gas, the active electrode may contain a reduction catalyst component. The inactive electrode contains a component that suppresses activity of the reduction catalyst component. An electrode component of the inactive electrode may be composed exclusively of a component that suppresses activity of the reduction catalyst component. Also, the active electrode and the inactive electrode may contain a predetermined electrode component and solid electrolyte component in combination. Preferably, a solid electrolyte component carries an electrode component.
In order to detect the concentration of combustible gas (particularly, HC or CO), the active electrode may contain an oxidation catalyst component. The inactive electrode contains a component that suppresses activity of the oxidation catalyst component. That is, the inactive electrode has the function of causing dissociation or recombination of oxygen molecules, and functions to suppress a combining reaction (burning reaction) of a combustible gas component and oxygen. The catalytically active electrode has the function of causing dissociation or recombination of oxygen molecules, and functions as an oxidation catalyst, which accelerates a combining reaction (burning reaction) of a combustible gas component and oxygen. An electrode component of the inactive electrode may be composed exclusively of a component that suppresses activity of the oxidation catalyst component. Also, the active electrode and the inactive electrode may contain a predetermined electrode component and a solid electrolyte component in combination. Preferably, the solid electrolyte component carries an electrode component.
Preferably the active and inactive electrodes are both located in the same cavity and are both exposed to substantially the same atmosphere.
Preferably, the active electrode contains one or more elements selected from the platinum group, which includes Pt, Rh, Pd, Ir and Ru. Preferably, the active electrode further contains Ag. If Ag is added to the platinum group element it increases the activity of the platinum group electrode.
Preferably, the inactive electrode contains one or more of Au, Ni, Co, Cr, Fe, Mn, Cu, Ti and Zn as the element for suppressing the activity of the electrode. In particular, Au or Cu are presently preferred as they work best at making the electrode inactive.
In order to detect NOx gas or combustible gas, the active electrode preferably assumes the form of a porous electrode and contains one or more platinum group elements as a main electrode component, while ZrO2 is added in an amount of approximately 10-20 wt % with respect to the electrode component. The inactive electrode assumes the form of a porous electrode and contains one or more platinum group elements as a main electrode component and approximately 10 wt % transition metal element(s) to suppress catalytic capability of the platinum group element, while ZrO2 is added in an amount of approximately 10-20 wt % with respect to the electrode component. The platinum group element(s) and the transition metal element(s) may be contained in the electrode in the form of an alloy formed of these elements only or an alloy formed of these elements and other components. Particularly, when NOx is to be detected, an element of higher NO dissociation capability (higher catalytic capability), such as Rh, is preferred as a component of the active electrode. When combustible gas is to be detected, the amount of transition metal added to the inactive electrode is preferably increased, since combustible gas is more likely to undergo a catalytic reaction (burning reaction) on the electrodes than in the case of detection of NO. Preferably, when combustible gas is to be detected, voltage applied to the electrodes is lowered, or sensor temperature is lowered.
Preferably, the oxygen-concentration-sensing electrode or the oxygen concentration reference electrode and the inactive or active electrode are implemented in the form of a common electrode, thereby further simplifying gas sensor structure.
The oxygen-concentration-sensing electrode, the oxygen concentration reference electrode, and the electrodes of the oxygen pump cell may be implemented entirely in the form of a Pt porous electrode. This is in order to impart to the electrodes a reversible catalytic function (catalytic function related to oxygen dissociation) in relation to a dissociation reaction of oxygen molecules for injecting oxygen into the solid electrolyte layers on which the electrodes are formed, and a recombination reaction of oxygen to cause the solid electrolyte layers to release oxygen. Preferably, the electrodes are formed such that a portion exposed to the interior of the cavity portion is implemented in the form of an Au porous electrode, while an opposite portion (the other portion) is implemented in the form of a Pt porous electrode. The Au porous electrode is catalytically inactive with respect to reaction of oxygen and a component to be detected, such as methane, while exhibiting a sufficient catalytic function related to oxygen dissociation with respect to operation of the oxygen sensor cell and the oxygen pump cell.
A ZrO2 solid solution containing Y2O3 or CaO is a typical material for a solid electrolyte layer that constitutes the oxygen sensor cell (having the oxygen-concentration-sensing electrode and the oxygen concentration reference electrode), the oxygen pump cell, and the oxygen concentration cell. Alternatively, the solid electrolyte layer may be formed of a ZrO2 solid solution containing an oxide of an alkaline earth metal or rare-earth metal. ZrO2, which is a base component of the solid electrolyte layer, may contain HfO2. The solid electrolyte layer may contain partially stabilized and/or stabilized ZrO2, CeO2, HfO2, and ThO2. A stabilizer may be selected singly or in combination from the group consisting of, for example, CaO, MgO, and rare-earth oxides (e.g., Y2O3, La2O3, and Gd2O3). Preferably, yttria partially stabilized zirconia powder (YSZ) is used as a stabilizer. Other stabilizers or other solid electrolytes may also be used.
Preferably, the cavity portion is defined between laminated solid electrolyte layers; specifically, between the solid electrolyte layer of the oxygen pump cell and the solid electrolyte layer of the oxygen sensor cell or oxygen concentration cell, and communicates with a subject gas atmosphere through a porous layer that serves as a diffusion passage. The cavity portion may be partially enclosed with an insulating layer. The porous layer that constitutes the diffusion passage reinforces the gas sensor to thereby prevent or suppress warpage or expansion of the gas sensor. Also, by employing the porous layer, contaminant particles, such as soot or oil mist, contained in exhaust gas become unlikely to enter, for example, the cavity portion, thereby preventing or suppressing deterioration of the electrodes, which would otherwise result from adhesion of contaminant particles. Preferably, the porous layer is an alumina-based porous ceramic layer having a number of communication pores formed therein.
In order to lead out outputs from the electrodes, electrode lead portions are formed that extend in the longitudinal direction of the gas sensor toward one end portion (a base end portion at which a sensor element is mounted) and that are electrically connected to the electrodes. The electrode lead portions are connected to leads at the base end portion for external connection. Preferably, the electrode that is in direct contact with a subject gas, such as exhaust gas, is covered with a porous protective film of, for example, alumina, spinel, zirconia, or mullite.